1. Filed of the Invention
The present invention pertains to a spectral stretching and control device for high peak power pulse lasers, as well as to a frequency-drift amplification chain comprising such a spectral stretching and control device.
2. Description of Related Art
The production of pulse lasers, of titanium-doped sapphire type (Ti:Sa), with very high peak power makes it necessary to control very wide spectra so as to decrease the pulse durations on output from the amplifying chain.
Two phenomena greatly limit the production of lasers of this type. The first is of a practical nature and relates to the significant bulkiness of the temporal stretching devices (Öffner type stretcher) making it possible to pass the spectral band. The second is of a physical nature and involves the spectral constriction and shift occurring in the amplifying medium.
A conventional solution for replacing the Öffner stretcher is to use an optical fiber, but recompression is made difficult because of significant spectral aberrations. As regards spectral constriction, the most commonly used solution consists in pre-compensating, at the start of the chain (before the so-called regenerative or multi-pass amplifier), the spectral deformation. This filtering-based solution has the drawback of limiting the extraction efficiency of the amplifiers and is all the less effective the larger the number of passes through the amplifiers.
Currently, lasers providing very high peak powers (of the order of a terawatt or more) for very brief times (of the order of a few fs) are of the frequency-drift amplification type (termed CPA, i.e.: Chirped Pulse Amplification). These lasers are based on the use of a wide spectrum, pulse stretching, amplification and recompression of the pulses thus stretched. Typically, these lasers are Ti:Sa chains which have an oscillator spectrum of from 5 to 100 nm, for compressed pulse durations of from 150 to 10 fs. The ability of an amplification chain to maintain a correct spectrum directly influences the ability of the laser to work with short pulses. The spectral constriction induced by the amplifiers is therefore a key factor for obtaining short-duration performance. Likewise, large deformation of the spectrum, for example asymmetric, will disturb the temporal shape and impair the operation of the laser.
The solution commonly used to temporally stretch the pulses before amplification is based on the Öffner stretcher. Its configuration is well known and makes it possible notably to minimize the spectral aberrations (see for example: G. Chériaux, P. Rousseau, F. Salin, J.-P. Chambaret, B. Walker, L. F. Dimauro: “Aberration free stretcher Design for ultrashort pulse amplification” Opt. Lett 21, 414-1996). The main limitation resides in the fact that, to stretch wide spectra, it is necessary to use optics of large dimensions. Even though solutions exist for limiting the bulkiness of this optical system (see French patent 2 834 080), these solutions are not entirely satisfactory. Specifically, the proposed solutions consist in working on the −1 order of the grating. It is thus possible to decrease the bulkiness of the stretcher for constant stretch. Öffner stretchers are nevertheless bulky and require precise alignment of the angles and length of the afocal setup (distance between concave and convex mirror of the afocal setup). Modification of a parameter of the stretcher acts moreover directly on the way in which the pulse will be recompressed.
In an Öffner stretcher, the pulse duration obtained at output depends on the parameters of the stretcher (focal length of the mirrors, number of lines of the gratings, angle of incidence) but especially on the spectral width of the pulse that is to be stretched. A parameter called the stretching factor and expressed in ps/nm is generally defined. This factor can vary from a few units, to a few tens. For an incident pulse of 100 nm, a stretching factor of 2 to 3 is sufficient to amplify the pulse to several hundred mJ. A smaller factor can be applied if the amplification is in the region of an mJ.
In CPA chains, the amplifiers used are of the type with n passes of the beam through the amplifying medium. When n is small (less than 10) the geometric multi-pass configuration is generally used. The pump laser dispatches a pulse into the crystal and the pulse beam to be amplified is thereafter dispatched into an amplifier stage in which it performs n passes through the laser crystal so as to optimize the extraction in terms of energy. FIG. 1 diagrammatically depicts a multi-pass amplifier of this kind, which essentially comprises a crystal 1 (for example Ti:Sa) receiving, from an input mirror ME, input pulses at an angle differing from the normal to its incidence surface, and several reflecting mirrors M1 to M7 disposed on either side of the crystal 1 so as to cause the beam to pass through the crystal at various angles of incidence, the last mirror M7 reflecting this beam to the output via an output mirror MS.
When a large amplification factor is sought, it is necessary to increase the number of passes and the configuration of FIG. 1 is no longer applicable. The configuration generally used is then the regenerative amplifier, an exemplary embodiment of which is shown diagrammatically in FIG. 2. This type of amplifier makes it possible to readily achieve some thirty or so passes.
The system represented in FIG. 2 comprises a crystal 2 disposed, with a Pockels cell 3, in an optical cavity closed by two mirrors 4, 5 and pumped by a pump 6. A polarizer 7, disposed in the cavity, makes it possible to tap off a part of the intra-cavity beam, the tapped-off beam passing through a half-wave plate 8, a reflecting mirror 9 and a Faraday rotator 10 at the output of which a semi-transparent mirror 11 reflects it back towards the use (beam Eout). Moreover, the polarizer 7 makes it possible to inject an external beam Ein into this cavity.
In both cases (FIGS. 1 and 2), the gain of the amplifier may be written:
      E    OUT    =                    J        SAT            .      S      .              ln        (                                                            J                STO                                            J                SAT                                      ⁢                          (                                                ⅇ                                                            E                      in                                                              SJ                      SAT                                                                      -                1                            )                                +          1                )              |  
JSTO being the stored fluence available for the gain in the medium (the crystal) and JSAT the saturation fluence of this medium. This is the classical equation from the theory of Frantz and Nodvick.
The table below contains a few examples of values of JSAT for various laser materials:
MaterialsJsat in J/cm2Spectral rangeDyes~0.001J/cm2VisibleExcimers~0.001J/cm2UVNd: YAG0.5J/cm21064 nm Ti: Al2O31.1J/cm2800 nmNd: Glass5J/cm21054 nm Alexandrite22J/cm2750 nmCr: LiSAF5J/cm2830 nm
In the small-signal regime, with JIN<<JSAT, the gain relation can be approximated with:
  G  =                    E        OUT                    E        IN              =          ⅇ              (                              J            STO                                J            SAT                          )            
The shape of the gain curve of the above-described amplifiers being close to a Gaussian, on each pass through the medium, a constriction of the spectrum due simply to the gain will be observed.
The curve of FIG. 3 shows a typical exemplary gain in a Ti:Sa crystal as a function of wavelength, this curve being centered on the wavelength of 800 nm.
As a result of the amplification in this laser medium, a gain which is non-uniform as a function of wavelength will therefore be applied to an input signal of limited spectrum, the effect of which is to cause an alteration: the spectral constriction. The example of FIG. 4 illustrates this effect, which is accentuated with the number of passes through the amplifier. The curve of the input signal as a function of its wavelength and the curves of the signal after 4, 10 and 30 passes through the crystal, respectively, have been represented in this FIG. 4. The effect becomes very significant when considering the case of a regenerative amplifier (30 passes for example).
It will be noted that when the input signal possesses a spectrum that is non-centered with respect to the maximum of gain of the medium, the spectral constriction is accompanied by a shift effect which tends to return the signal to the maximum gain spike.
To compensate for these effects, a pre-distortion of the input signal is usable by active or passive filtering at the price of a decrease in the efficiency of the laser. Indeed, the filters used have efficiencies of the order of 50% since they act (cut off) spectrally at the energy maximum.